Chewy Peanut Butter-Chocolate Slices

Line an 8" square pan with foil. Melt peanut butter and 2/3 C. sweetened condensed milk in microwave (on High for 2 1/2 minutes); stir and microwave 30 seconds longer. Stir in 1/2 tsp vanilla and spread mixture in lined pan, spreading evenly with fingers. Repeat with remaining sweetened condensed milk and chocolate chips, melting in microwave at High 1 1/2 minutes; add remaining vanilla. Spread over peanut butter mixture. Chill until firm. Lift block out of pan with edges of foil and cut into 20 pieces (4 columns by 5 rows) with pizza cutter (easiest way). Store in airtight container in refrigerator.